Structures Compounds that Control Rhythms

Rhythms can be generated by exogenous sources or can be produced endogenously by a physiological "clock".2-6 The daily light/dark cycle is the best-known exogenous source that can set and regulate daily rhythms. This cycle is primarily conveyed to the nervous system by the retina. The site of the biological "clock" is the suprach-iasmatic nucleus of the hypothalamus6 and this nucleus receives input from the retina to synchronize the endogenous rhythms with the exogenous light/dark cycle (Figure 12.2). Another site that contributes to timing mechanisms is the pineal gland. In mammals, this gland receives rhythmic neural information from the suprachiasmatic nucleus and produces a hormone, melatonin, in response. Melatonin (5-methoxy-#-acetyltryptamine) can modulate the biological "clock" as well as provide a hormonal signal to other cells to produce coordinated rhythmic responses.7 Melatonin is currently used to reset circadian rhythms as a jet lag treatment and is used to induce sleep. It is also being used experimentally as an anticonvulsant to control or modify the rhythmic nature of seizures.8,9

FIGURE 12.2

Artistic depiction of the neuroanatomic pathway that conducts photic information from the retina to the suprachiasmatic nucleus (the biologic clock) and the multisynaptic pathway that conveys this information to the pineal gland. The pineal gland releases the hormone, melatonin, on a rhythmic basis, with the peak occurring during the dark period. Melatonin is able to act on the suprachiasmatic nucleus and regulate "clock" function.

LIGHT ^

LIGHT ^

FIGURE 12.2

Artistic depiction of the neuroanatomic pathway that conducts photic information from the retina to the suprachiasmatic nucleus (the biologic clock) and the multisynaptic pathway that conveys this information to the pineal gland. The pineal gland releases the hormone, melatonin, on a rhythmic basis, with the peak occurring during the dark period. Melatonin is able to act on the suprachiasmatic nucleus and regulate "clock" function.

INTERMEDIOLATERAL CELL COLUMN

INTERMEDIOLATERAL CELL COLUMN

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